1. Field of the Invention
The present invention relates to a gain equalizer flattening a spectrum of input light in a predetermined wavelength range, and an optical amplification apparatus amplifying signal light in the predetermined wavelength range.
2. Related Background Art
An optical amplification apparatus amplifies signal light inputted through an input terminal and outputs the amplified signal light from an output terminal, and, for example, is used in a WDM (Wavelength Division Multiplexing) optical communication system. Such optical amplification apparatus is required to flatten an entire gain spectrum in a predetermined wavelength range. However, a gain spectrum of an optical amplification medium generally used in the optical amplification apparatus (for example, an Er-doped optical fiber) is not flat.
That is, the optical amplification apparatus comprises a gain equalizer having a loss spectrum with the same shape as the gain spectrum of this optical amplification medium, and flattens the entire gain spectrum synthesizing the gain spectrum of the optical amplification medium and the transmission spectrum of the gain equalizer, in the predetermined wavelength range. As this gain equalizer, the use of a gain equalizer including a long-period grating is described in Japanese Patent Application Laid-Open No. 2002-82235, and the use of again equalizer including a slanted grating is described in Ashish M. Vengsarkar, et al., “Long-Period Fiber Gratings as Band-Rejection Filters”, Journal of Lightwave Technology, Vol. 14, No. 1, pp. 58-65 (1996), and Isabelle Riant, et al., “36 nm Amplifier Gain Equalizer Based on Slanted Bragg Grating Technology for Multichannel Transmission”, Suboptic 2001, P4.3.10 (2001).
A transmission spectrum of a filter such as the long-period grating or the slanted grating has a shape as shown in FIG. 1. FIG. 1 shows transmission spectrums of a filter having a wide bandwidth (corresponding to filter A) and a filter having a narrow bandwidth (corresponding to filter B), respectively. As can be seen from this figure, in the transmission spectrum of the filter A, the bandwidth where a transmittance is −0.1 dB or less (hereinafter referred to as minimum bandwidth) is 10 nm. On the other hand, the transmission spectrum of the filter B has a minimum bandwidth with 6 nm. In general, the gain equalizer has a transmission spectrum with such shape is constituted by a plurality of filters connected in series and each having a different minimum transmittance, a different minimum transmittance wavelength and a different minimum bandwidth. FIG. 2 shows the target transmission spectrum Tt of the gain equalizer and the actual transmission spectrum Ta of the gain equalizer with which a plurality of filters are applied, and FIG. 3 shows an error between the target transmittance and the actual transmittance.